Glass and/or mineral fibers are widely used for thermal and/or acoustic insulation. In the case of glass fibers it is common practice to chop continuous filament material into short lengths (staple fibers), thereafter forming a mat from the staple fibers produced, or simply packing the staple fibers into a supporting member. Thus staple fibers are packed into automotive muffler or silencer casings, into cavity walls, or are incorporated into sandwich panels for use in building construction.
The mechanical chopping of glass filaments into staple fibers requires high speed rotating machinery; it may also expose workers to the physiological effects of staple fibers which are usually harsh, spiky and abrasive. In the case of automotive muffler or silencer casings the handling of staple glass fibers is a particular problem. It is difficult to accurately meter loose fibers entrained in an airstream, which is the usual mode of fiber transfer, especially where only a limited time is available to fill each casing, as on an automated production line for silencers. In this specification and the claims that follow, silencers and mufflers are used interchangeably.
It is well known that a continuous glass fiber roving or sliver can be bulked by exposure to a highly turbulent airstream prior to deposition in a container as a fleece without breaking the filaments. It has been proposed in EP-A-0091413 that this process should be used to fill automotive silencer casings with bulked, continuous filament glass fibers, using suction applied through the casing to effect deposition of the appropriate quantity of glass fiber.
The process just described employs a conventional textile bulking or texturing jet as a means of exposing a continuous filament roving to the action of a highly turbulent airstream. It also uses a separate cutter device operable to sever the roving on completion of each silencer filling operation.
Common to known processes for filling silencer casings with glass fibers is the problem of achieving uniform bulk density of the filled material. As the casing fills up it is progressively more difficult for air to escape through the fibrous mass, even using suction and an/or an auxiliary airflow. Also, the material is both very bulky and very resilient, so it tends to spring back towards the outlet of the bulking jet. This progressively affects the quality of the bulking operation; it eventually slows down the rate of delivery from the jet, by virtue of progressively occluding the jet outlet. It also results in the last material supplied to a casing being of significantly lower bulk density than the first material supplied, to the point where it is even impossible to transfer the filled casing to further processing stages such as the installation end caps, because the filled material tends to overflow out of the end of the casing. EP-A-0091413 discloses a process for filling a silencer casing, but only from one open end thereof. Such a process is effective for roughly half of the commonly used types of absorptive silencer. There are, however, other very commonly used types of absorptive silencer where the process just referred to is ineffective and/or inefficient. For example, there are `straight-through` silencers, the automated production of which includes the step of fitting both end caps at once. For these, it is normal to use a glass fiber preform made in situ around a length of perforated exhaust gas duct to locate the latter duct inside the casing prior to affixing the end caps. Preform manufacture is an essential, extra step in this particular process. There are also silencers which have two separate fiber-filled absorptive regions either side of a reactive element comprising baffles in an intermediate fiber free volume. The absorptive regions may be of different shapes and/or sizes, but once again it is normal to fit both end closures at the same time.
It is an object of the present invention to provide an improved process and apparatus for filling a silencer casing with glass fibers.